Please consider this for any FAQ or I hope this may help others.
If your pi-hole is connected to the network, for example, 192.168.1.0/24 and you you have hosts on a different network for wireless (192.168.2.0/24), home automation (192.168.100.0/24) and VPN connections (192.168.200.0/24) and so on, then you need to go to the pi-hole admin portal, sign in as admin, navigate to settings> DNS> select expert, and modify the Interface settings to select the option “respond only on interface $interface” for example eth0, or something else depending on what VPS or physical equipment you are running pi-hole on.
If you do not change this setting then even if DNS traffic reaches pi-hole from another client, as verified by tcpdump, wireshark and other network tools, responses will not be sent back to the client from pi-hole, since the client is on a different network. The default setting is “allow only local requests” meaning in this example only hosts on 192.168.1.0/24 will receive a reply.